There is proposed a system in which an air-fuel ratio sensor and an O2 sensor are disposed on an upstream side and a downstream side of an exhaust gas purifying catalyst, respectively, and which is configured to perform main F/B control and sub F/B control, wherein the main F/B control controls a fuel injection amount on the basis of an output value of the air-fuel ratio sensor such that an air-fuel ratio of an exhaust gas converges to a target air-fuel ratio, and the sub F/B control controls the target air-fuel ratio such that an output value of the O2 sensor converges to a target value (e.g. refer to Patent document 1).
According to an air-fuel ratio control apparatus for an internal combustion engine disclosed in the Patent document 1, the target air-fuel ratio is feedback-controlled to set a constant correction amount per unit time with respect to an oxygen adsorption amount of the exhaust gas purifying catalyst even if an intake air amount changes. It is therefore possible to prevent a significant deviation of an exhaust gas purifying catalyst atmosphere from a purification window even in a large intake air amount state, resulting in an improvement in emission.
Incidentally, Patent document 2 discloses an apparatus configured to change a feedback gain more significantly, if a duration in which output voltage of the O2 sensor is out of an area between a rich side threshold value and a lean side threshold value exceeds a predetermined time length.